Chemical mechanical polishing (CMP) is often used to remove material from a surface of a workpiece to provide a relatively smooth, flat surface on the workpiece. For example, CMP may be used in the formation of semiconductor devices and magnetic hard disks to provide a smooth, flat surface for subsequent processing of the devices or disks.
Material on a workpiece surface is removed using CMP by placing the workpiece surface in contact with a polishing surface and moving the workpiece and the polishing surface relative to each other. A slurry, including abrasive particles and/or chemicals that react with the material on the workpiece surface to dissolve the material, may also be placed in contact with the workpiece surface to assist removing a portion of the material.
During CMP processing, it is often desirable to remove a predetermined amount of material from the surface of the workpiece or leave a predetermined amount of material on the surface of the workpiece by removing excess material. Device or disk performance may vary in accordance with the thickness of a particular layer remaining on the surface of the workpiece. Thus, it is often desirable to mitigate variation of material removal or remaining material thickness across the workpiece surface to increase yield and/or performance of devices or disks, such that the remaining film thickness is near an optimal value across the surface of the workpiece.
Variation of material removal across a workpiece surface may be a function of several parameters or factors. For example, variation of material removal may be a function of, among other things, relative movement (e.g., direction and speed) between the workpiece and the polishing material, amount of slurry present at the workpiece surface, type of polishing material, material to be removed from the workpiece surface, tilt of a workpiece with respect to the polishing surface, pressure applied between the workpiece and the polishing surface, compression of the polishing material, and the number of workpieces cumulatively polished on a polishing material surface.
CMP modeling may be used to predict how various process parameters affect the variation of material removed across the workpiece surface. Modeling predictions may then be used to select polishing parameters to reduce variation of material removed across the surface of the workpiece, increase workpiece material removal rate, or a combination thereof. In addition, CMP modeling parameters may be used to predict how new materials (e.g., workpiece material, slurry material, polishing material, or any combination thereof) will affect material removal rates and variation of material removed from the workpiece surface.
Unfortunately, effects of various polishing parameters on workpiece material removal variation are not well understood, in part, because measurements of factors that affect material removal variation are difficult to obtain. For example, it is difficult to measure pad compression, wafer tilt, and other factors that affect material removal variation while polishing the workpiece.
An example of a CMP machine configured to measure effects of polishing parameters and use the measured effects to control a polishing process is disclosed in U.S. Pat. No. 5,868,896, issued to Robinson et al., on Feb. 9, 1999. The machine disclosed in the '896 patent includes a pressure sensor interposed between a workpiece and a carrier configured to guide the workpiece across a polishing surface. The pressure sensor generates a signal corresponding to the contour of the workpiece surface in contact with the polishing surface. The signal from the pressure senor is coupled to a controller, which in turn is coupled to an actuator on the carrier. To reduce variation in an amount of material removed from a wafer surface, the actuator increases or decreases pressure applied to portions of the wafer in response to the sensed pressure.
Although the CMP apparatus disclosed in the '896 patent discloses apparatus to measure pressure and adjust carrier force in response to the measured pressure, the apparatus does not provide for a mechanism to measure process parameters such as pad compression and wafer displacement. Accordingly, improved method and apparatus for measuring effects of various CMP polishing parameters are desired.